Fundamental semiconductor physics

In the parlance of solid-state physics, semiconductors (and insulators) are defined as solids in which at 0 K (and without excitations) the uppermost band of occupied electron energy states is completely full. It is well-known from solid-state physics that electrical conduction in solids occurs only via electrons in partially-filled bands, so conduction in pure semiconductors occurs only when electrons have been excited--thermally, optically, etc.--into higher unfilled bands.

At room temperature, a proportion (generally very small, but not negligible) of electrons in a semiconductor have been thermally excited from the "valence band," the band filled at 0 K, to the "conduction band," the next higher band. The ease with which electrons can be excited from the valence band to the conduction band depends on the energy gap between the bands, and it is the size of this energy bandgap that serves as an arbitrary dividing line between semiconductors and insulators. Semiconductors generally have bandgaps of approximately 1 electron-volt, while insulators have bandgaps several times greater.

When electrons are excited from the valence band to the conduction band in a semiconductor, both bands contribute to conduction, because electrical conduction can occur in any partially-filled energy band. The current-carrying electrons in the conduction band are known as "free electrons," though often they are simply called "electrons" if context allows this usage to be clear. The free energy-states in the valence band are known as "holes." It can be shown that holes behave very much like positively-charged counterparts of electrons, and they are usually treated as if they are real charged particles.

One of the most important characteristics of semiconductors is the fact that both positive and negative change carriers exist. When an electron is excited from the valence band to the conduction band, a positive "hole" is left in the valance band while the excited electron becomes a negative charge carrier in the conduction band. Another electron in a nearby position in the valence band is then free to fill the positive hole, leaving a hole in the spot where that electron originated. In this fashion, the hole appears to move about the valence band of the crystal and is functionally a positive charge carrier.

If a semiconductor is pure and if it is unexcited by an input like an electric field, it allows very little current to pass through it, and it is practically an insulator. The main reason that semiconductors are so useful is that the conductivity of semiconductors can be manipulated by addition of impurities (doping), by introduction of an electric field, by exposure to light, or by other means. For example, CCDs, the primary unit of digital cameras, rely on the fact that semiconductor conductivity increases with exposure to light. Transistor operation, which will be discussed below, depends on the fact that semiconductor conductivity can be increased by the presence of an electric field.

Current conduction in a semiconductor occurs via free electrons and holes. Holes aren't real particles; in a sense that requires some knowledge of semiconductor physics to understand, a hole is the absence of an electron. Nevertheless, this absence, or hole, can be treated as a positively-charged counterpart to the negatively-charged electron. Indeed, the precise meaning of "free electrons" also requires a background in semiconductor physics to understand. For descriptive ease, "free electrons" are often simply denoted "electrons," but it should be understood that the majority of electrons in a solid, which aren't free, do not contribute to conductivity.

If a semiconductor crystal is perfectly pure, with no impurities, and it is held at a temperature near absolute zero with no excitations (e.g. electric fields or light), it will contain no free electrons and no holes, and thus will be a perfect insulator. At room temperature, thermal excitations produce some free electrons and holes in pairs, but most semiconductors at room temperature are insulators for practical purposes.